Acetaminophen, a mild analgesic and a antipyretic drug is both hepatotoxic and nephrotoxic at high doses. Under therapeutic dose conditions tissue glutathione is known to protect the liver by reacting with quinoneimine form of acetaminophen. It is also suggested in the literature that glutathione can detoxify the acetaminophen phenoxyl radical. In this work we have investigated the mechanism of detoxification of the acetaminophen phenoxyl radical. Acetaminophen was oxidized by horseradish peroxidase system to generate the acetaminophen phenoxyl radical. This radical reacts with both glutathione and ascorbate. In the presence of glutathione both the phenoxyl radical and the disulfide radical anion were detected. In the presence of ascorbate, only the ascorbyl radical was detected. These observations suggests that ascorbate rather than glutathione is more likely to be involved in the detoxification mechanism in vivo. The rate of formation of the disulfide radial anion was also measured by the oxygen-consumption measurements, and the GSSG formed was determined by optical measurements. The carcinogenic action of 4-nitroquinoline N-oxide has been attributed to hydroxyaminoquinoline N-oxide and its oxidation products. Therefore we studied the reaction of hydronitroxide quinoline N-oxide radical generated by horseradish peroxidase, with glutathione and ascorbate. We found that ascorbate reduces the hydronitroxide free radical completely, but glutathione reacts very slowly. The disulfide radical anion was not detected in this case, thus showing that ascorbate is a better free radical reducing agent than glutathione, and may be more important in deactivation in vivo.